Current anti-predation rules are designed to detect and prevent actions that are only taken to drive out a rival. We evaluate the performance of these rules in a simple entry game. We find that the rules used by competition authorities fail to encourage sustained competition in the market. Moreover, despite the rules an inefficient incumbent cannot be replaced by a more efficient entrant unless the difference in efficiency is extreme. One reason for these failures is that incumbents choose a strategic response to the legal environment. Large incumbents, for instance, crowd the product space. This is detrimental to welfare and consumer surplus.